Wide dynamic range sensor signal processing method &amp; circuitry for analog and digital information signals

ABSTRACT

A sensor provides an analog sensor output signal that is first converted by a A/D converter, followed by a digital signal processor having a digital signal output that is converted to an analog signal by way of a D/A converter so as to provide an analog information output signal indicative of a parameter quantity intended to be sensed. An analog dither signal and/or a digital dither signal is provided for modulating the A/D converter input signal and/or D/A converter input signal, respectively, separately, or in combination, to enhance dynamic range accuracy of the resultant analog information output signal representative of the quantity of the parameter intended to be sensed.

RELATED APPLICATION

This application claims the benefit of priority pursuant to 35 USC 119of provisional patent application Ser. No. 60/352,890 filed 30 Jan.2002, the disclosure of which application is hereby incorporated in itsentirety by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to sensor signal processingmethods and circuitry for analog sensors, and more particularly to asensor signal processing method and circuitry for sensors and meteringsystems employed in power line current and voltage sensor applicationswith enhanced accuracy over a wide dynamic range.

BACKGROUND OF THE INVENTION

Optical sensors are commonly employed in a wide variety of applicationsincluding among others, inertial navigation sensors systems where anoptical sensor is responsive to rotation thereof, and optical currentand voltage sensors where the optical sensor is responsive to electricand/or magnetic fields. Each of these optical sensors employs an opticalbeam or light wave that propagates along a specific optical path. Eachof these optical sensors generally includes light detector circuitry andsignal processing circuitry for sensing a specific parameter thataffects the optical beam along the optical path. More specifically, thepropagating light wave is affected by the intended parameter intended tobe monitored or sensed, for example rotation of the optical path forinertial sensors, electric fields for voltage sensors, and magnetic orelectric fields for current sensors, and the like. The characteristicbehavior of the light wave or optical beam, for example velocity, phase,and/or polarization may be affected by the intended parameter to besensed in accordance with well-known and established physicalprinciples.

Commonly, optical sensor systems of the types described employ aphoto-detector for obtaining an indication of the instantaneousintensity of one or more optical beams, either separately or incombination, impinging on the photo-detector. In turn the photo-detectoroutput signal is signal processed by a signal processing circuit forderiving therefrom an indication of the parameter intended to bemeasured—the parameter affecting the optical beam, e.g., the magnitudeof the of the change in velocity, phase, and/or polarization inaccordance with well known and established principles. Further, thesignal processing circuit may also include one or more outputs that maybe utilized as a drive signal for affecting the optical path and therebybe incorporated into a closed-loop feedback signal processing sensorscheme for obtaining the indication of the parameter intended to bemeasured as is in accordance with well known and established principles.

A relatively new application of optical sensors is high voltage powerline monitoring systems. These systems require highly accurate currentand voltage sensors in which the sensors are physically immersed in thebrutal electrical environment of high voltage power lines where the linevoltage is in the order of hundreds of Kilo-volts and power line currentmay be in the range of 1.0 to 5000.0 amps. The latter wide dynamiccurrent range needed to be measured by current sensors is a severeproblem in the power line application.

The wide dynamic range requirement may be understood by considering thatpower plants coupled to an electrical power line grid may be operativein one scenario providing peak current output during heavy summertimeuse due to air conditioning equipment, i.e., 5000 amps. In anotherscenario, the power plant may be in an idling mode where the power plantacts as a small load receiving a small amount of current, or is puttingout a small amount of current, say, in the order of 1.0 amp.

This wide dynamic range accuracy requirement has commonly necessitatedthe use of multiple sets of instrumentation to measure power linecurrents during the aforesaid scenarios. For example, transformersemployed for obtaining current measurements may include multiple tapswhere each tap may be coupled to two or more metering instrumentationsystems or meters.

Optical current sensors, and more particularly fiber optic currentsensors, are now employed in power-line applications for measuring widedynamic range current flowing through the power lines. Principles ofthese types of optical sensors are taught in U.S. Pat. No. 5,644,397,entitled, “Fiber Optic Interferometric Circuit And Magnetic FieldSensor,” issued to James N. Blake. Disclosed therein are both Sagnacinterferometric and in-line embodiments for constructing an opticalcurrent sensor. Various improvements thereof are also disclosed, amongothers, in U.S. Pat. Nos. 5,696,858, 5,987,195, 6,023,331, and6,122,415, all issued to James N. Blake.

Briefly, fiber optic current sensors as disclosed in the aforementionedpatents work on the principal that a magnetic field produced by acurrent to be sensed affects the polarization properties of a sensingfiber in the vicinity of the magnetic field through the Faraday effect.The change in the polarization properties of the sensing fiber can beprobed in several different ways. Common ways include injecting linearlypolarized light and later analyzing the rotation of its polarizationstate after exiting the sensing region, or measuring the relativevelocities of right and left hand circularly polarized light waves thattravel through the sensing region using a Sagnac or in-lineinterferometer technique. The sensor configuration includes an opticalexit port for permitting an “affected” optical beam to exit the opticalcircuit and impinge upon a photo-detector positioned at the terminus ofthe optical circuit. In all cases, the current to be sensed causes lightintensity fluctuations of the exiting optical beam, and in turn causesthe photo-detector output signal to exhibit fluctuations related to thelight intensity fluctuations thereby providing a signal which may beprocessed by a signal processing circuit to provide a signal indicativeof the current intended to be sensed.

The fluctuations of the light intensity falling on the photo-detectormay be processed by analog circuitry to produce an analog outputrepresentative of the current being sensed. However, it is often moredesirable to digitally process the light intensity fluctuations. Digitalprocessing is often more desirable because: (i) digital output is oftenmore desirable for following subsystems; (ii) more accuracy may beobtained with digital signal processing because the higher processingpower available in the digital domain allows for complexcharacterization, (iii) it is much cheaper to carry out complex signalprocessing in the digital domain rather than in the analog domain, and(iv) wide dynamic range signals may be more accurately handled in thedigital domain.

Generally, the first step required for processing the photo-detectorsignal in the digital domain is to convert the signal using an A/Dconverter. After the signal has been processed, it may be converted backto the analog domain using a D/A converter. This conversion back to theanalog domain is often required for current and voltage sensors as thesesensors may be married to secondary or receiving devices (such asmeters, relays, and recorders) which in the power industry often haveanalog system front-ends.

It should be noted that these secondary devices also have a wide-dynamicrange requirement. For example, consider a power meter for measuring theproduct of sensed current and voltage where the current ranges, asbefore between 1.0 and 5000.0 amps. Commonly such power meters includeanalog-to-digital converters and digital signal processing for derivingthe desired information, for example watt-hours. As indicated earlier,multiple sets of instrumentation, i.e., multiple meters, may be employedto obtain accurate information depending on the current.

The absolute accuracy of these types of optical sensors as well as thesecondary devices, i.e., meters, may be compromised by the A/D and D/Aconverters employed therewith. This may happen because of thequantization and non-linearities present in the A/D and D/A converters.As an example, consider what happens in an optical current sensor thatis made to measure currents as low as ±1 amp, and as high as ±5000 amps.Suppose a 12-bit A/D converter is used to convert the photodetectoroutput signal to a digital signal. These 12 bits (4096 distinct levels)have to describe a 10,000 amp range, or on average, each bit representsabout 2.5 amps. Thus the 1 amp signal falls within 1 least significantbit (LSB). Normally, some noise will exist in the detectedphoto-detector signal that serves to “dither” the detected signal aroundseveral LSB's. This noise may be used to overcome the quantization errorassociated with the signal being comparable to or less than an LSB.However, the overall system accuracy is often not good enough even whenthe signal with noise spans several LSB's. This is so since the bitspacing for these several LSB's may not be representative of the overallbit spacing for the A/D converter. If these bits are closer togetherthan the overall average bit spacing, then the final output signal willread relatively higher than a large signal which uses many of the bits;and, if these bits are further apart than the average bit spacing, thenthe final output signal will read relatively lower than a large signalthat uses many of the bits.

The same problem as just described for the A/D converter also applies tothe D/A converter. Larger converters such as 16-bit or higher A/D or D/Amay be used to reduce the magnitude of this problem, but to meet thedemanding specifications of wide dynamic range optical sensors withpresent day (typically 16 bit or less) A/D and D/A converters, there isa need for a signal processing circuit that diminishes the effects ofbit non-linearities in these types of optical sensors, including voltageand current sensors, requiring large dynamic range.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a signal processingcircuit having wide dynamic range accuracy for optical sensors.

An object of the present invention is to provide a signal processingcircuit having wide dynamic range accuracy for power line applicationsecondary devices responsive to power line current measurements.

An object of the present invention is to provide a signal processingcircuit for the photo-detector output signal of an optical sensor.

An object of the present invention is to provide a signal processingcircuit having wide dynamic range accuracy for analog sensors.

An object of the present invention is to provide a signal processingcircuit for a photo-detector output signal of an optical sensor wherethe photo-detector output signal is converted to a digital signal,processed, and returned to an analog representation of a parameterintended to be sensed by the optical sensor.

In accordance with present invention, a signal processing circuit isprovided for an optical sensor having a photo detector output signal.The signal processing circuit is configured to be responsive to thephoto-detector output signal where the photo-detector output signalcontains information of a parameter quantity intended to be sensed bythe optical sensor, for example, optically sensed power line current.The optical sensor provides an analog sensor output signal that is firstconverted by a A/D converter, followed by a digital signal processorhaving a digital signal output that is converted to an analog signal byway of a D/A converter so as to provide an analog information outputsignal indicative of a parameter quantity intended to be sensed. Ananalog dither signal and/or a digital dither signal is provided formodulating the A/D converter input signal and/or D/A converter inputsignal, respectively, separately, or in combination, to enhance dynamicrange accuracy of the resultant analog information output signalrepresentative of the quantity of the parameter intended to be sensed.

The present invention is particularly directed to optical current andvoltage sensors, as well as any optical sensor having a photo-detectoroutput signal intended to be digitally signal processed to provide anoutput signal representative of the quantity of the parameter intendedto be sensed.

The present invention is also directed to any measurement systemresponsive to a an analog sensor input signal which may be digitallyconverted for subsequent signal processing to derive selectedinformation or subsequent conversion back to an analog signal to providean output signal representative of the quantity of the parameterintended to be sensed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the signal processing circuit inaccordance with the present invention.

FIG. 2 depicts an analog dither signal in accordance with the presentinvention.

FIG. 3 depicts discrete digital values of a dithered analog sensoroutput signal.

FIG. 4 is a schematic block diagram of another embodiment of a signalprocessing circuit in accordance with the present invention.

FIG. 5 is a schematic block diagram of another embodiment of a signalprocessing circuit in accordance with the present invention.

FIG. 6 is a schematic block diagram of another embodiment of a signalprocessing circuit in accordance with the present invention employing afield programmable gate array.

FIG. 7 is a schematic block diagram of another embodiment of a signalprocessing circuit in accordance with the present invention.

FIG. 8 is a schematic block diagram of another embodiment of a signalprocessing circuit in accordance with the present invention particularlyillustrating a power line metering and monitoring application.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated in FIG. 1 is a schematic block diagram of a signalprocessing circuit 100 in accordance with the present invention. By wayof example, an optical sensor is depicted in FIG. 1 as an opticalcurrent sensor 110 where sensor 110 includes a current sensor head 112that provides a light propagation path 114 through which a light wave116 propagates there along. Sensor head 112 is intended to be inproximity to an electrical conductor 115 through which a currentI_(x)(t) flows therethrough and is intended to be measured as describedin the aforementioned patents, all of which are incorporated herein byreference thereto, and in accordance well known and establishedprinciples.

Associated with the optical sensor 110 is a photo-detector 120responsive to one or more light waves associated with light wave 116 andcontains the necessary information, e.g., velocity, phase, and/orpolarization, to provide an indication of the current flowing throughconductor 115. Photo-detector 120 is illustrated as being coupled to aoptical propagation path 118, e.g., an optical fiber, for directing alight wave 122 associated with light wave 116 so as to impinge uponphoto-detector 120. In turn, photo-detector 120 provides an analogoutput signal P(t) representative of the intensity of the light wave 122impinging on photo detector 120. Although not shown, photo-detector 120generally may include low noise amplification or pre-amplificationcircuitry so as to produce a signal sufficient to be recognized by A/Dconverter 150 in a manner as will subsequently described.

Signal processing circuit 100 in accordance with the present inventionincludes an analog dither signal generator 130, analog summing circuit140, analog-to-digital (A/D) converter 150, digital signal processor160, digital-to-analog (D/A) converter 170, and output filter circuit180.

In accordance with the present invention, an “analog signal dither” isutilized to linearize the resultant digital output information from anA/D converter that the A/D converter may inherently corrupt an analoginformation signal intended to be converted into a digital informationsignal. Similarly, a “digital signal dither” may be utilized tolinearize the resultant analog output information signal from a D/Aconverter that the D/A converter may inherently corrupt a digitalinformation signal intended to be converted into the analog informationsignal.

In accordance with the present invention, power line measurement andmonitoring equipment, particularly illustrated on FIG. 8, commonlyutilize analog output signals from analog current and voltage sensorswhich need to signal processed, preferably by digital techniques forobtaining selected information. Such power line measurement andmonitoring equipment commonly include A/D and D/A converters that maycorrupt the corresponding analog input signal information. As indicatedearlier, the wide dynamic range requirements may exacerbate theaforesaid corruption yielding diminished accuracy. Further, as indicatedin FIG. 1, such converters are commonly employed with particular use ofcurrent sensors employing optical sensors, as well as other types ofsensors, in power line applications as depicted in FIG. 1. Suchconverters are also employed in other power line application secondarydevices and/or instrumentation.

Referring again to FIG. 1, dither signal generator 130 is intended toprovide an analog dither signal AD(t) having known characteristics. Forexample, dither signal generator 130 may provide a time varying analogsignal having a known signal waveform, amplitude, and frequency as willbe subsequently described with reference to FIGS. 2 and 3. The analogdither signal AD(t) need not be periodic, but preferably has knownproperties. On the other hand, dither signal generator 130 could takethe form of an analog signal noise source having known or detectableproperties.

Analog dither signal AD(t) is summed together with the photodetectorsignal P(t) by way of analog summing circuit 140, and provides an outputsum signal S(t) as an input to A/D converter 150. The output of the A/Dconverter 150, namely DS(t), is provided as the primary input signal todigital signal processor 160 for performing digital signal processing ofsignal DS(t) as desired. In turn, digital sensor signal processor 160provides a digital signal output DQ(t) representative of the parameterintended to be sensed by the optical sensor 110 including a dithersignal component. Digital signal output DQ(t) is coupled to D/Aconverter 170 for converting digital signal output DQ(t) into analogoutput signal AX(t) which, in turn, may be signal filtered by filtercircuit 180 for providing a filtered analog output signal FAX(t)representative of I_(x)(t) where “FAX” nomenclature represents thefiltered analog signal AX(t) representative of I_(x)(t).

It should be noted that digital signal processor 160 may take on manyforms to derive an indication of the parameter intended to be sensed,and depends upon the information contained in the intensity variationsof the detected light wave by photo-detector 120. Examples of suchprocessing are indicated in the aforementioned patents, albeit in analogform. Further, digital signal processor 160 may be constructed by way ofhardware, software and/or firmware for achieving the intendedinformation. An example of digital signal processor 160 may be a“FPGA”—Field Programmable Gate Array well known in the art andparticularly depicted in FIG. 6 as will be subsequently described.

Operation of the signal processing circuit 100 in accordance with thepresent invention will now be described. Analog dither signal AD(t) iscombined with photo-detector output signal P(t) to form analog signalS(t) for subsequent presentation as the input to A/D converter 150.Analog signal S(t) is essentially signal P(t) modulated by the dithersignal AD(t). In this way, local bit spacing errors of A/D converter 150are averaged out over a dither signal period of signal AD(t). Thewaveform of the exemplary analog dither signal AD(t) as illustrated inFIG. 2 may take many forms, including noise, and be effective for theintended purpose of modulating the digital output signal DS(t) of A/Dconverter 150.

In an exemplary embodiment of the invention, analog dither signalgenerator 130 is configured so a to provide a triangle waveform signalAD(t) as particularly illustrated in FIG. 2. In turn, the output of A/Dconverter 150 may have discrete digital values, as depicted in FIG. 3,for subsequent signal processing by digital signal processor 160. It hasbeen found that the choice of a triangular waveform tends to give equalweighting to all the bits covered by the analog dither signal AD(t).Further, it has been found that it is effective for the amplitude of theanalog dither signal AD(t) to be such that it causes a few percentvariation of the total bit range of the A/D converter 150 for each cyclethereof. In an exemplary embodiment of the invention, analog dithersignal generator 130 is configured so that the peak-to-peak signal ofAD(t) is sufficient to dither the A/D converter 150 over 1/16 of thedigital range of A/D converter 150 at 2 kHz.

As indicated earlier, the embodiment illustrated in FIG. 1 is such thatthe analog dither signal AD(t) is interpreted by the digital signalprocessor 160 as a component of the sensed current I_(x)(t), but indigital form, and again shows up in the output AX(t) which may be thenfiltered by filter 180 to remove the analog dither component so toprovide signal FAX(t) indicative of the parameter intended to be sensed.

As a further enhancement to the signal processing circuit of FIG. 1,digital signal processor 160 may include an optional digital ditherstripper 190 for digitally stripping the digital signal output of A/Dconverter 150 either prior to digital signal processing by processor 160or thereafter. Synchronization signal line 192 represents thesynchronization between the analog dither signal AD(t) and the digitaldither stripper 190. This function, namely digitally stripping the addedanalog dither signal before the A/D converter 150 may be accomplished bya wide away of techniques well known to those skilled in the art. Insome circumstances, it may be desirable to not completely strip thedither signal contained in signal DQ(t) as will subsequently bedescribe, particularly for subsequent secondary devices responsive tosignal FAX(t).

In FIG. 4, thereshown is an alternate arrangement of a signal processingcircuit in accordance with the present invention where like componentshave retained the same numeral designations. The analog dither signalgenerator 130 and analog summing circuit 140 of FIG. 1 are eliminated inFIG. 4. The photo-detector output signal P(t) is coupled directly intoA/D converter 150. As before digital signal processor 160 processesdigital signal DS(t) so as to derive a digital sensor signal DQ(t)indicative of the parameter intended to be sensed. Digital signal outputDQ(t) of digital signal processor 160 is digitally combined by way ofdigital summing block 410 with a digital dither signal DD(t) provided bydigital dither signal generator 420, and provides a digital sum signalDX(t). In turn, the digital output signal DX(t) is converted to ananalog signal AX(t) which is filtered by analog filter 180 to remove thedigital dither signal DD(t) provided by way of digital dither generator420, and provide analog output signal FAX(t) representative of theparameter intended to be sensed.

Digital dither signal DD(t) serves to average bit usage in the outputD/A converter 170 to produce analog signal AX(t). Since this digitallyinduced dither signal DD(t) appears in the output signal AX(t) of D/Aconverter 170, signal AX(t) is desirably filtered if it is unacceptablefor it to be present. The digital dither signal frequency is preferablyvery much higher than the signal frequency bandwidth of the inputcurrent I_(x)(t) so that it may be effectively filtered out by filter180 in the form of a simple low-pass filter.

In FIG. 5, thereshown is an arrangement of a signal processing circuit500 that employs novels aspects of the present invention as alreadydescribed with reference to FIGS. 1 and 4. In FIG. 5, like components ofthose described with reference to FIGS. 1 and 4 have retained the samenumeral designations. In the arrangement of signal processing circuit500 illustrated in FIG. 5, (i) the photo-detector output P(t) isdithered by the analog dither signal AD(t) prior to being digitized byA/D converter 150 (like FIG. 1); and (ii) the output DQ(t) of digitalsignal processor 160 is digitally dithered by digital dither signalDD(t) prior to being converted to an analog signal AX(t) by A/Dconverter 170 (like FIG. 4) followed by and filtered by filter 180 toproduce signal FAX(t).

As before with reference to FIG. 1, the analog dither signal may bestripped away before or after digital signal processing as taught withreference to blocks 190 and signal 192 (not shown in FIG. 5). The signalprocessing circuit of FIG. 5 takes advantage of analog dither signalAD(t) to enhance the accuracy of the signal from the A/D converter 150,and takes advantage of the digital dither signal DD(t) to enhance theaccuracy of the signal from the D/A converter 170. Filter 180 may beconstructed, as indicated earlier to filter out, at least in part,either or both the analog dither signal AD(t) as well as the digitaldither signal DD(t).

In FIG. 6, thereshown is an arrangement of a signal processing circuit600 that employs novels aspects of the present invention as alreadydescribed with reference to FIGS. 1, 4 and 5. In FIG. 6, like componentsof those described with reference to FIGS. 1, 4 and 5 have retained thesame numeral designations. In the arrangement of signal processingcircuit 600 illustrated in FIG. 6, (i) digital signal processor 160 isdepicted as a field programmable gate array (FPGA) indicated by numeraldesignation 660; and (ii) the analog dither signal generator 130 isreplaced with a D/A converter 630 responsive to the digital signalDAD(t) provided as an output from FPGA 660 on signal line 632. Inaccordance with the circuit construction as depicted in FIG. 6, the FPGAmay easily provide digital dither stripping of signal DS(t) internal tothe FPGA without any additional circuitry. Although not shown, a digitaldither signal DD(t) may likewise be provided as an output of the FPGAwith selected characteristics being the same or different than that ofsignal DAD(t).

Illustrated in FIG. 7 is another aspect of the present inventiondepicting a secondary device as earlier described for power lineapplications or the like. Thereshown is a secondary device 700 includingan analog dither signal generator 790, signal conditioning circuit 710,analog summing circuit 792, A/D converter 794, and digital signalprocessor 795 providing an output signal DMQ(t). The construction andoperation of device 700 is similar to that already described withreference to FIG. 1. Signal conditioning circuit 710 may optionally bedesired to provide filters, transformers, and pre-amplifiers, asdesired.

As is well known in the art of power line measuring and monitoringequipment, a signal representative of the power line current indicatedby signal FAX(t), that may be determined by current sensor systems otherthan optical sensors, is illustrated as an input to secondary device700. Secondary device may be a meter or recording device as desired,generally including circuits and systems that require conversion of thesensed current signal into digital form for subsequent processing.Accordingly, an A/D converter 794 is employed to convert the analogsignal into a digital signal for subsequent signal processing,recording, monitoring, and/or the like. As described earlier, device 700needs to have wide dynamic range enhanced accuracy. Commonly, such widedynamic range enhanced accuracy for secondary power line applicationdevices is obtained, as indicated earlier, by way of multiple sets ofinstrumentation to achieve the intended accuracy.

In accordance wit the present invention, secondary devices for powerline monitoring and measuring devices may be achieved by way of applyinganalog dither MAD(t) to the analog input signal FAX(t) by way of analogdither signal generator 790 and summing circuit 792 before being coupledto A/D converter 794 in a manner as already described. In turn, asbefore, the output DM(t) of the A/D converter 794 is digitally processedby digital signal processor 795 so as to obtain the intended digitalsignal output DMQ(t). Although not shown, the embodiment of FIG. 7 mayalso include filtering and dither signal stripping as desired andpreviously described.

FIG. 8 illustrates a simple power meter 800 as an example of one suchsecondary device for power line applications. Like components of FIG. 7have retained the same numeral designation in FIG. 8. As is customary,device 800 utilizes an analog signal representative of current I_(x)(t),and an analog signal representative of voltage V_(x)(t). Like FIG. 7,signals I_(x)(t) and V_(x)(t) may be operated on by optional signalconditioning circuitry as indicated by blocks 810 and 820, respectively.The voltage signal V_(x)(t) is converted to a digital voltage signal byway of A/D converter 894, and the current signal I_(x)(t), along withthe analog dither signal MAD(t), is converted to a digital signal by wayof A/D converter 794. In turn, digital signal processor 795, responsiveto the outputs of A/D converters 794 and 894 may be employed to arriveat a variety of digitally processed data, for example power, i.e., theproduct of voltage times the current. Further, processor 795 may provideother information such as time associated data as well as watt-hourinformation, and the like.

It should be noted that application of the dither signal techniqueachieves the desired wide dynamic range without the need for multiplesets of instrumentation. In such a metering application as illustratedin FIG. 8, the voltage sensor signal does not need wide dynamic rangesince it is most likely a predictable value over a narrow range ofvalues. In contrast, the current sensor signal needs highly accuratewide dynamic range since it may make take on values from low—severalamps, to high—several thousand amps.

As set forth in the accompany description of the invention, a method hasbeen taught to achieve wide dynamic range of current sensors andsecondary devices, such as meters for power line applications.Specifically, analog sensor signals are combined with an analog dithersignal before being analog-to-digitally converted for subsequent digitalsignal processing. Similarly, digital output signals may be combinedwith a digital dither signal before being digital-to-analog signalconverted for subsequent use by analog secondary signal processing, forexample metering equipment.

Although the present invention had been particularly described withemployment of optical sensors, any analog sensor that is intended to bedigitally converted is within the true spirit and scope of the presentinvention.

Further, it should be recognized that although circuit components havebeen indicated in the drawings by way of separate blocks performingseparate functions in order to enhance understanding of the invention,such functions may be more or less incorporated into other componentsserving the intended functions. For example, the digital signalgenerator and digital summing components may be incorporated into amulti-function digital signal processor which may be constructed by wayof hardware, firmware, software, and the like as is well known to thoseskilled in the art in order to achieve the intended functions as setforth in the accompanying claims.

While the present invention has been particularly shown and describedwith reference to the accompanying figures, it will be understood,however, that other modifications thereto are of course possible, all ofwhich are intended to be within the true spirit and scope of the presentinvention. Various changes in form and detail may be made thereinwithout departing from the true spirit and scope of the invention asdefined by the appended claims.

1. Power line measurement equipment for deriving selected informationfrom selected current and voltage sensor information associated with apower line, and including at least one analog sensor producing an analogsensor output signal containing information associated with at least oneof current and voltage intended to be sensed, the power line measurementequipment comprising: an analog signal generator for providing an analogdither signal; a signal summing means for summing said analog sensoroutput signal and said analog dither signal and providing an analog sumsignal representative of the sum of said analog dither signal and saidanalog sensor output signal; an analog-to-digital converter responsiveto said analog sum signal for providing a digital sum signalrepresentative of said analog sum signal wherein said analog-to-digitalconverter has a known number of bits forming said digital sum signal;and a digital signal processor responsive to said digital sum signal forderiving a digital sensor signal containing information related to saidat least one of current and voltage.
 2. The signal processing circuit ofclaim 1 wherein said digital signal processor further includes means fordigitally subtracting said analog dither signal such that said digitalsensor signal is substantially devoid of any dither signal component. 3.The signal processing circuit of claim 1 further comprising adigital-to-analog converter for converting said digital sensor signal toan analog sensor output signal.
 4. The apparatus of claim 3 furthercomprising an analog filter means for filtering said analog sensoroutput signal and providing a third analog sensor signal indicative ofat least one of current or voltage intended to be sensed by said opticalsensor.
 5. The power line measurement equipment of claim 1, wherein saidcurrent is between −5000.0 and 5000.0 amps.
 6. Power line measurementequipment for deriving selected information from selected current andvoltage sensor information associated with a power line, and includingat least one analog sensor producing an analog sensor output signalcontaining information of a associated with at least one of current andvoltage intended to be sensed, the power line measurement equipmentcomprising: an analog-to-digital converter responsive to an analog inputsignal including at least a component of said analog senor outputsignal, and for providing a digital information signal representative ofsaid analog input signal; a digital signal processor responsive to saiddigital information signal for deriving a digital sensor signalcontaining information related to said at least one of current andvoltage; a digital-to-analog converter responsive to said digital sensorsignal for providing an analog information signal representative of saiddigital sensor signal; and at least a selected one of, an analog signalgenerator for providing an analog dither signal for dithering saidanalog input signal provided as said input to said analog-to-digitalconverter, and a digital dither signal generator for digitally ditheringsaid digital sensor signal provided as said input to saiddigital-to-analog converter.
 7. The signal processing circuit of claim 6wherein said signal processor includes a field programmable array. 8.The signal processing circuit of claim 6 wherein: said signal processorincludes a field programmable array, including a secondary digitaldither signal; and said analog dither signal generator is a digitalanalog converter for converting said secondary digital dither signal tosaid analog dither signal.
 9. The power line measurement equipment ofclaim 6, wherein said current is between −5000.0 and 5000.0 amps.